1. Field of the Invention
The invention is concerned with electric fuses.
2. Description of the Prior Art
The design of fuses for the protection of electrical circuits against current overload involves consideration of a number of fuse characteristics depending on the type of circuit to be protected. A first fuse characteristic, the so-called current rating, is defined as the strongest current which a fuse will permit to pass indefinitely without blowing. A second fuse characteristic variably known as time lag, clearing time, fusing speed, or simply speed is defined as the time which elapses between the application of a current overload and the blowing of the fuse. The use of a slow fuse, i.e., a fuse with a relatively long time lag, may be indicated in applications such as the protection of electromechanical equipment where short duration switching currents exceeding the current rating of the fuse should leave the fuse intact. A particular design of such a purposely slow fuse is described in "Electric Fuses" by H. W. Baxter, published by Edward Arnold & Co., 1950. The fuse, disclosed by Baxter on pages 38-40 has a current rating of 0.4 A. and can carry a 20 percent current overload for one minute before blowing. While slow fuses may also be useful for the protection of radio sets having large capacitors, the protection of delicate solid state electronic equipment is preferably ensured by fast fuses, i.e., by fuses with fast response to current overload. When comparing fuses it has to be borne in mind that clearing time of a fuse is a function of current overload.
Additional general concerns in the design of fuses are the corrosion resistance of the fuse element and the prevention of arcing between terminals upon fusing of the fuse element. A special concern with the mechanical strength of the fuse element arises with indicating fuses, i.e., fuses in which the fuse element is spring loaded and in which the spring energy, upon blowing of the fuse, becomes available, for example, to close an alarm circuit. Indicating fuses are particularly suited for applications where the quick identification of a blown fuse in a large array of fuses is important; for example, such fuses may be used for protection of complicated equipment such as electronic computers and switching systems.